This invention relates to methodology focused on making a unique, plural-component, composite-material highway (especially concrete-highway) dowel bar structure having features which successfully address, and resolve, key, serious, and interrelated, functional-reliability, componentry cost, and highway lifecycle economic problem issues that have for years been associated with highway dowel-bar technology, and relationally, with certain aspects of highway functionality longevity.
The architecture of this methodology is described herein in a manner intentionally intertwined with a companion description of the unique dowel bar which is fabricated in accordance with it.
Dowel bars are ubiquitous in concrete highway construction. They take the form of elongate, cylindrical, high-shear-strength, smooth-exterior rods, typically formed of conventional, structural steel (non-stainless), which are employed in multiple numbers, typically at 1-foot lateral-spacing intervals, as shear-bridging, slab-to-slab load-transfer components at each separation interface (joint) which exists between adjacent, travel-direction-oriented, highway slabs that are usually formed of poured concrete, and preferably, in many instances, of what is known in the art as high-performance concrete. Their most important functions are to furnish appropriate load-transfer shear strength between adjacent slabs while at the same time (a) accommodating slab expansion and contraction (with slight, but recurrent, slab-joint openings and closings) due to ambient thermal changes, and additionally (b) minimizing the phenomenon known as slab faulting at the joints between adjacent slabs. They are conceived as simple structures, sometimes differently formed, in relation to efforts to achieve improvements over the use alone of conventional structural steel, of materials, and combinations of materials, such as epoxy-coated structural steel, zinc-coated structural steel, solely stainless steel, stainless-steel-jacketed conventional structural steel, and purely single-unit plastic composite materials—all intended to perform the above-mentioned, important “highway-performance-support” functions flawlessly for long periods of time.
However, for a number of costly and disappointing reasons outlined below, they do not do this. Intent and reality historically have not matched successfully, and the resulting hunt for necessary, important, and fundamental dowel-bar improvement, has for years been, and is still, ongoing.
It is our belief that a dowel bar which is made in accordance with the practice/methodology features of the present invention changes this situation, and that the “hunt” just mentioned may now be substantially over.
Further regarding aspects of the background, dowel-bar setting which has triggered the conception and reduction to practice of the present methodology invention, dowel bars are employed in the multi-millions in concrete highway construction, both in the United States and in numerous other countries. In such highway construction, the usual, adjacent highway-pavement slabs that lie, edge-to-edge, along a normally constructed highway lane may typically have a length of about 15-feet, and a width of about 12-feet. Between each, successive, adjacent lane slab, dowel bars, at about 1-foot lateral intervals, are included as interconnecting structures.
Because of what are currently very predictable, current-dowel-bar-materials' wear and corrosion characteristics, and due to the facts (1) that dowel bars, and the confronting, concrete faces of their associated highway slabs, are both exposable at all inter-slab, concrete highway joints, and (2) that dowel bars, during their operative lives, are subjected to substantial cyclic shear loads in the multi-millions even over relatively short periods of time—a condition which is very relevant to the durabilities and configurations of the intended, protective coatings that are furnished for various types of coated steel bars (mentioned below), they are not, and cannot comfortably be considered to be, permanent, or even satisfactorily and relatively long-lasting components of concrete highway structure in many applications. A result is the need for too-frequent dowel-bar (and associated concrete-slab) replacement—an extraordinarily costly need.
Despite many improvement-focused efforts launched in the past to minimize the necessity for, and the attendant frequencies of, highway dowel-bar replacement, these efforts have not been entirely successful. For example, and considering commonly understood corrosion and wear issues that are associated with known dowel-bar constructions, typical, widely-used, conventional steel dowel bars are obviously subjected to environmental-conditions rusting and corrosion, and do not possess characteristics which inherently help to resist these issues. Among past “improvement” attempts, purely stainless steel dowel bars, while less vulnerable, though not immune, to corrosion, nevertheless are eventually corrodible, and, very significantly, are extremely expensive as items, per se, and often prohibitively so.
Other improvement fabrication approaches taken in the past, such as coating traditional structural steel dowel bars with, for example, thin layers of epoxy or zinc, is expensive, and, for a variety of reasons, has not worked particularly well. Apart from the expense issue, and considering specifically structural matters, epoxy coatings are typically applied by spraying (or by some other suitable, surface-application technique) to long “over-lengths” of conventional, structural-steel, dowel-bar core material, with the resulting, long, core-coated product then crosscut selectively to achieve finished dowel bars of the desired lengths. The crosscut, end areas are usually then simply left uncoated, with the result that exposed steel exists at those cut ends which are thus fully exposed to all nearby, corrosive environmental conditions.
Where epoxy coating is applied completely over already pre-length-cut steel dowel-bar cores, i.e., where no post-coating cross-cutting is involved, or under those circumstances where cross-cut ends are in fact subsequently coated with epoxy, the resulting all-over epoxy coating effectively “flows”/extends as a material-continuum over the sharp-edged surface discontinuities that exist, at each end of a dowel bar, between the cylindrical surface of the core steel bar material and the end surfaces of this material—a condition which thus exposes the coating material, at these sharp discontinuity regions, to very high-stress mechanical conditions, with respect to which cyclic loading of dowel bars, in their installed use conditions in concrete highway slabs, easily triggers regional stress fractures in a coating—fractures which then expose the core steel material at those vulnerable locations to corrosion-attack environmental conditions.
Another main concern involving epoxy-coated steel bars is that typical rough handling and installation of these bars damages (i.e., cracks, chips, etc.) and noticeably compromises the protective effectiveness of the employed epoxy material.
Zinc coating of conventional structural steel bar material produces dowel bars that also present a problematic issue. Zinc coating is employed as a corrosion-accepting, sacrificial layer over steel, inhibiting steel corrosion, yes, but in its sacrificial role becoming extremely roughly surface-textured, a condition interfering with the desired capability of a dowel bar to offer smooth-outside-surface sliding contact with surrounding concrete to accommodate necessary, temperature related expansion and contraction of concrete slabs.
Solid, fibre-reinforced, plastic-based, composite-material dowel bars have been proposed and tried, but, as individuals, they do not offer the shear strength of steel, and thus do not function well as comparably robust, slab-to-slab load transferors. Accordingly, where such plastic, reinforced dowel bars are selected for use, they are usually employed (a) in greater numbers, and (b) with closer lateral spacings, than are characteristic for steel dowel bars, and at least for this reason involve greater expense. Stainless-steel-jacketed, conventional steel dowel bars are quite expensive, and notwithstanding stainless jacketing, in some instances, disappointingly, also corrodible.
With these negative, expense and corrosion-driven, performance-longevity matters in mind, one of the most significant issues involving extensive dowel-bar use in concrete highway construction is that, when dowel-bar replacement becomes necessary—and frequently too often so—the materials and labor-time and -wage costs, and the related downtime associated with the need for construction-site-preparation and the subsequent pouring of new highway concrete slab material (because of the fact that, where failed dowel bars are located, the surrounding concrete highway material must be demolitioned), are extraordinary.
In this setting, there is clearly a need for an improved highway dowel bar which offers, along with impressive-performance shear strength, great resistance to corrosion decay, with both of these qualities and characteristics collaboratively contributing to (1) significant reduction of replacement need, (2) appreciable lengthening of replacement intervals where replacements are required, and in the bargain, (3), impressive minimizing of highway lifecycle, etc. costs. These considerations, while important in all concrete highway settings, are particularly important considerations where a highway is to be what is known as a high-performance highway built with high-performance/long-life concrete.
Accordingly, fabricated by practice of the methodology of the present invention is a plural-component, composite-material highway dowel bar including an elongate, high-shear-strength, cylindrical core, preferably, although not necessarily, formed of steel, and an elongate, fibre-reinforced plastic-resin jacket fully surrounding and protectively covering the entirety of the core, and exposed to no high-stress-point discontinuities, such as surface-edge discontinuities, anywhere within the overall dowel-bar structure. The jacket includes, as it results from practice of the associated invention methodology, an elongate, fiber-reinforced, cylindrically tubular, plastic-resin sleeve which circumsurrounds and is bonded to the core. The jacket sleeve, which is intentionally made longer than the inside steel core, has opposite ends that extend beyond the associated opposite ends of the received core, thus to define, together with the inwardly disposed core ends, a pair of spaced, single-open-ended, elongate, cylindrical, sleeve end wells, or simply end wells. Two, elongate, cylindrical, fiber-reinforced, plastic-resin sleeve end plugs (also referred to herein simply as plugs), each having a diameter and a cross section matching that of the steel core, fill the sleeve end wells, and are united in the overall dowel-bar structure by a bonding mechanism, such as one or more of those mentioned immediately below, which firmly secures them to the sleeve in the end wells. These plugs, together with the receiving sleeve, protectively seal the side and opposite ends of the core. The central, high-shear-strength core and the two end plugs in each dowel bar are collectively referred to as a core assembly. The outside of the sleeve is smooth. Tenacious bonding between the sleeve and the inside core assembly may include any one or more of several bonding mechanisms, including sleeve shrinkage, normal mill scale roughness of steel, associated, of course, just with the steel core, molecular inter-material forces with both the steel core and the end plugs, and other.
Thus, and very importantly, nothing about the protective jacket, and its making in accordance with the present invention, involves any condition of jacket-material “folding” around and over the sharp edge discontinuities that are present adjacent the ends of the central steel core. This important condition exists chiefly because of the fact that the sleeve in the jacket is formed, pursuant to practice of the present invention, with a length which is greater than that of the steel core, that the sleeve extends beyond each of the opposite ends of the core, and that sleeve-received and bonded end plugs having diameters and cross sections matching that of the core close the opposite ends of the sleeve to complete the core-protective jacket.
The jacket's included sleeve is fibre reinforced in plural, resin-embedded, fibre-differentiated, circumferentially-adjacent circumferential layers of fibres which take the forms of (1) elongate, linear, plural-elongate-fibre-including, glass-fibre roving material whose included fibres substantially parallel the long axis of the dowel bar, (2) continuous-glass-fibre mat material, and (3), glass-fibre veil material. The sleeve end plugs are formed preferably of roving-fibre-reinforced plastic resin, wherein the included fibres are linear and also disposed substantially parallel the long axis of the dowel bar.
The methodology of the invention proposes a pultrusion-based, dowel-bar making process including, in general terms, (a) preparing an elongate core train (preferably substantially horizontally disposed) possessing endo-abutting, longitudinally alternating (1) elongate, cylindrical, high-shear-strength cores, and (2) shorter, elongate, cylindrical and matching-cross-section resin and fiber-reinforced, pre-pultruded sleeve end-plug blanks, (b) using the core train as a longitudinally moving mandrel, pultrusion-forming a resin and fiber-reinforced sleeve continuously and bondedly around the core train so as to produce a pultrusion-result intermediate product, and (c) cross-cutting the pultrusion-result intermediate product at the longitudinal center locations of the sleeve end plugs to form the final dowel bars.
Pre-pultrusion, of the just-above-mentioned, pre-pultruded sleeve end plugs does not specifically, or necessarily, form a methodologic, procedural part of the present invention, and may be conducted in an entirely conventional manner. Such pre-pultrusion, however, is a very useful end-plug-making technique for assuring that the end plugs, in order to cooperate successfully in the dowel bar made by practice of the invention, are satisfactorily reinforced with throughout-distributed, elongate, linear fibres that substantially parallel the end plugs' long axes.
These and other features and advantages of, and offered by, the methodology of the invention will become more fully apparent as the detailed description of it which follows below is read in conjunction with the accompanying drawings.
Components, structures and positional relationships between elements presented in FIGS. 1-5, inclusive, are not drawn to scale.